Federal officials are instructing oil giant Royal Dutch Shell to conduct underwater monitoring to assess the origin of a 10-mile stretch of oil sheen that is located near two of its platforms in the Gulf of Mexico.

The US Coast Guard spotted the sheen Wednesday afternoon. On Thursday, the Bureau of Safety and Environmental Enforcement sent a helicopter to confirm the sheen, located about 130 miles southeast of New Orleans.

Where the sheen originated, its density, and composition are still not known. BSEE instructed Shell to send remote-operated vehicles to investigate nearby wells that Shell does not own but are permanently plugged, as well as surrounding areas in the Gulf that are known to produce natural seeps.

Eileen Angelico, chief of BSEE’s public affairs office, says results from those operations will be “expected in hours and not days.”

Shell released a statement Thursday that said it is “confident at this time that the sheen did not originate from” its two platforms, named Mars and Ursa, that collectively produce about 60,000 barrels of oil a day. It is not yet known if the company conducted chemical tests of the oil sheen to make that determination.

The company also confirmed it has “no well control issues associated” with drilling operations in the area. Hurricane Katrina severely damaged the Mars platform in September 2005; it resumed operations in 2006.

An oil sheen the size of the one discovered Wednesday could originate from a variety of sources: a platform, well, tanker, or natural seep. How to determine which of those is the culprit becomes more difficult the longer the clock is ticking, because of changes to the Gulf current and dissipation of the material itself, says Sam Bentley, an associate professor of geology and geophysics at Louisiana State University in Baton Rouge.

“The longer the lag between identifying of the material on the surface and linking it back to the source, the greater the uncertainty increases,” Professor Bentley says. “That doesn’t mean you can’t make an educated guess.”

Chemical fingerprinting samples of the sheen can help trace the composition of the oil back to a source, which is “the ideal scenario,” Bentley says. Samples can also determine if the oil is pristine or processed, which can rule out if it is vessel fuel.

Shell’s claim that their platforms did not produce the sheen is feasible due to the speed of Gulf currents. Ocean currents in deep water can move several miles per hour. In the 2010 oil spill at the Macondo well, which is considered the largest environmental disaster in US history, oil sheen was found at least 100 miles from the source.

Oceanographers can determine the area where sheens originate by back-calculating wind and ocean currents, Bentley says.

BP, the primary stakeholder in the Macondo disaster – the two-year anniversary of which is April 20 – holds a stake in both Shell platforms. Exxon Mobile Corp. and ConocoPhillips are partners in Ursa, according to Bloomberg.

The Mars platform operates in 2,900 feet of water; Ursa in 4,000 feet. Macondo, which is now permanently sealed, is located 5,000 feet below the surface.